The present invention relates to a magnetic separation apparatus. More particularly, the present invention is directed to a magnetic separation apparatus which can remove undesirable substances from a liquid to be treated, in which suspended solids, suspended organisms such as algae and fungi, and the like are contained, to thereby purify the liquid at a high rate, and also to a liquid purification apparatus using the magnetic separation apparatus.
Many of closed water areas such as lakes and rivers, reservoirs, and the like are advanced to be nutritious by inflow of domestic effluent or the like in associate with urbanization. As a result, multiplication of phytoplanktons has taken place to pollute the closed water areas. Occasionally, the problem arises that Mycrocystis (a state that phytoplanktons such as bluegreen algae (Cyanophyceae) excessively multiply, thereby forming a bluish green layer on a water surface) generate. In particular, in the event that a large amount of Mycrocystis generate in summer season, it is strongly demanded to develop a technology which can remove those at high rate with energy savings.
Conventionally, as a water purification apparatus for purifying raw water such as rivers or reservoirs, waste water, and the like by utilizing such a solid/liquid separation art, a magnetic separation apparatus having a two-stages magnetic separation step utilizing a magnetic attraction is proposed in JP-A-55-132684, and also a dewatering concentration method after separation is proposed in JP-A-55-61979.
The apparatus described in JP-A-55-132684 performs a magnetic treatment method comprising previously adding and dispersing ferromagnetic seeds into waste water in which most of materials to be treated are non-magnetic and diamagnetic materials, coagulating the materials to be treated and the magnetic seeds in the waste water by the action of a flocculent or the like, magnetically filtering off magnetic aggregates formed of the materials to be treated and the magnetic seeds, and separating the materials to be treated from the waste water. Here, the magnetic filtration method comprises magnetically sedimenting magnetic aggregates having a relatively large size in the first stage, and magnetically filtering magnetic aggregates having a relatively small grain diameter which have not been sedimented in the first stage within a predetermined retention time, in the second stage to obtain a treated water.
This method is a high gradient magnetic filter separation system in which a ferromagnetic filtration medium such as a ferromagnetic stainless wool in the magnetic filtration apparatus of the second stage is excited by an exciting medium such as an electromagnet or a permanent magnet from the outside, purification operation is temporarily stopped after accumulating a certain degree of magnetic aggregates on the ferro-magnetic filtration medium, reverse washing operation is conducted such that the magnetic aggregates accumulated on the ferromagnetic filtration medium are washed and separated using treating water, and the reverse washing water containing the magnetic aggregates is temporarily stored in a rinsing tank. This reverse washing water contains much treating water. Therefore, a step of again concentrating the magnetic aggregates from the reverse washing water is necessary, but its treatment method is not described at all. Further, there is no description regarding a high rate treatment method of the magnetic aggregates sedimented in the first stage. Thus, it is not clear how method should be performed to conduct a high rate purification operation as a whole apparatus.
On the other hand, JP-A-55-61979 discloses a high rate treatment method of a washing water containing magnetic aggregates after reverse washing in the high gradient magnetic separation method. This is, for example, a method using a rotating magnetic separation apparatus comprising disposing a rotating disc having permanent magnets disposed on the circumference of the disc, in a rinsing tank, and directly separating the magnetic aggregates from the washing water on the surface of the permanent magnets. However, this prior art involves the disadvantages that since all the magnetic aggregates are captured and stored in the high gradient magnetic separator, a separation portion is clogged with the magnetic aggregates within a short period of time, resulting in shortening a reverse washing operation period, and the purification operation is disturbed from a high rate operation. Further, since all the magnetic aggregates in raw water are contained in the washing water for reverse washing, a large amount of the magnetic aggregates must be separated through the rotating magnetic separation apparatus, making it difficult to conduct reverse washing water purification at a high rate.
However, with increasing a volume of a purification treatment amount of water treatment apparatus, a high current velocity of a treating water in a magnetic separation portion, continuous purification and concentration operations of reverse washing water become necessary. As mentioned above, the magnetic separation apparatus which captures and removes the magnetic aggregates by two stages depending on their sizes is advantageous for a high rate operation. However, the magnetic separator in the first stage must continuously remove the separated large-sized magnetic aggregates from a duct.
Further, in order to attempt a high rate operation as a purification apparatus including the operation up to concentration of reverse washing water, separation of the magnetic aggregates from reverse washing water must be conducted at a high rate.
Further, JP-A-60-244390 describes a technology in which magnetic particulates are mixed with raw water containing red tide in order to recover red tide or Mycrocystis present in hydrosphere, those are captured with a first magnetic filter, particulate substances captured (red tide planktons and ferromagnetic particulates are intermingled with each other) are recovered by washing the first magnetic filter, the particulates in which red tide planktons and ferro-magnetic particulates are intermingled are separated into the red tide planktons and the ferromagnetic particulates by a second magnetic filter, and the separated red tide planktons (red tide slurry) are dewatered and concentrated with a centrifugal separator.
In this method, after separating into the red tide planktons and ferromagnetic particulates, the red tide planktons (red tide slurry) are dewatered and concentrated with a centrifugal separator. In view of the fact that red tide planktons and Mycrocystis generated in lakes and rivers float on a water surface, their density is about 1 which is substantially the same as the density of water. Therefore, even if those are treated with a centrifugal separator, it is difficult to separate water and the red tide slurry on the theory of a centrifugal separator.
JP-A-55-61979 describes a technology in which magnetite and a flocculent are added to raw water as materials for separating solid substances and water contained in sewage and waste water, not red tide and Mycrocystis, those are separated into water and aggregates by magnetic separation, magnetite is further added to the solid substances to re-coagulate those, the re-coagulated sludge is adsorbed on magnets disposed on a rotating plate, the adsorbed sludge is scraped with a scraper, and the scraped sludge is placed in a heating furnace to heat the same therein, thereby separating into solid substances and magnetite.
This method involves the problem that since magnetite is further added to the aggregates obtained by the magnetic separation, and every aggregate containing magnetite is introduced into a heating furnace to dewater the aggregate and separate magnetite as a solid substance, magnetite in an amount more than the necessary amount is required to use, and a heating furnace for separating the solid substances and magnetite is required.